This invention relates to tracheal tube assemblies and to methods of assisting ventilation.
Tracheal tube assemblies having an outer tube and inner cannula are known. With such assemblies, the inner cannula is removed and replaced periodically when secretions have built up on the cannula to an extent that there is a risk of blockage. Tracheal tube assemblies are described, for example, in U.S. Pat. No. 3948274, GB 2056285B, GB 1099277, GB 125754, WO 90/04992, FR 2539998A, DE 72467, DE 1268313, EP 0107779A, U.S. Pat. No. 4817598, U.S. Pat. Nos. 3659612, 4009720, 3088466, 4315545, 2765792, 3169529, 3263684, 3334631, 3587589, 3688774, 3731692, 3889688, 3948273, 3973569, 3987798, 4033353, 4045058, 4235229, 4471776, 4593690.
The inner surface of tracheal tubes, in use, tends to accumulate a film of respiratory secretions and bacteria. This film can obstruct the bore of the tube and reduce gas flow along it. It has been found that the film can also act as a site for build up of bacteria in quantities sufficient to cause infection if dislodged from the tube and subsequently inhaled. The use of an inner cannula which is periodically removed and replaced, can reduce these effects but brings with it disadvantages. In particular, the thickness of the wall of the inner cannula will itself reduce the effective bore of the tube. This can be mitigated by making the external diameter of the inner cannula as close as possible to the internal diameter of the tube and making the cannula wall as thin as possible in order to maximize the diameter of the gas passage through the tube assembly. It is also important to prevent passage of secretions between the outside of the cannula and the inside of the outer tube. A close fitting inner cannula can, however, be difficult to insert because of friction with the bore of the tube. This can make the inner cannula prone to kinking, especially if the wall of the cannula is thin. This difficulty is aggravated by the fact that the inner cannula is preferably made of material to which secretions will cling without becoming dislodged into the bronchii and that these materials, such as PVC, tend to have a relatively high coefficient of friction. These materials also tend to be relatively flexible, so that the cannula wall must be made thick enough to prevent kinking on insertion.